BRAF gene is a gene encoding a type of serine threonine kinase constituting the RAS-RAF-MAPK pathway. Mutations in BRAF gene have been reported in various tumors. For example, the mutation wherein the valine at amino acid 600 is substituted with glutamic acid (V600E) is found in many cancer cells. BRAF having the V600E mutation is known to always activate the downstream signaling and cause a cell growth without extracellular stimulation.
For example, the BRAF gene having the V600E mutation is found in many colorectal cancers (5 to 15%) and melanoma (about 60%). Note that, in many cancers such as pancreatic cancer and colorectal cancer, mutations in a KRAS gene are found at high frequency. The KRAS protein is G proteins locally present on the inner surface of cell membrane. RAS such as KRAS activates RAF such as CRAF and BRAF, RAF sequentially activates MEK and MEK activates MAPK, thus forming a cascade (Non Patent Literatures 1 and 2). It is a rare case to contain both BRAF mutation and KRAS mutation as the BRAF mutation and the KRAS mutation are in the mutually exclusive relationship (Non Patent Literature 3).
Currently, for cancers with BRAF having the V600E mutation, a therapy by an inhibitor against BRAF having the V600E mutation is considered effective. Vemurafenib (PLX4032) and PLX4720 are known as such an inhibitor. However, cancer cells may acquire the resistance to these inhibitors by the continuous administration thereof which makes the therapy effects limited. Thus, for cancer cells having a mutation in the BRAF gene, more effective cell death inducing agents and growth suppressing agents replacing these inhibitors have been in demand.
Glutathione-S-transferase (GST), one of the enzymes catalyzing the glutathione conjugation, is known as an enzyme conjugating a substance such as a drug with glutathione (GSH) into an aqueous substance. GST is, based on the amino acid sequence, classified representatively into 6 types of the isozyme, α, μ, ω, π, θ and ζ. Of these, the expression of GST-π (glutathione S-transferase pi, also referred to as GSTP1) particularly has been increasing in various cancer cells and is indicated to have been a possible factor of the resistance to some anticancer agents. In fact, it is known that when an antisense DNA or a GST-π inhibitor against GST-π is allowed to act on a drug resistant cancer cell system overexpressing GST-π, the drug resistance is suppressed (Non Patent Literatures 4 to 6). Further, in a recent report, when siRNA against GST-π is allowed to act on a GST-π overexpressing androgen-independent prostatic cancer cell line, the growth thereof is suppressed and the apoptosis is increased (Non Patent Literature 7).
Additionally, GST-π is known to form a complex with c-Jun N-terminal kinase (JNK) and inhibit the JNK activity (Non Patent Literature 8). Further, GST-π is known to be involved with the S-glutathionylation of proteins associating with the cell stress responses (Non Patent Literature 9). Furthermore, GST-π is known to contribute to the protective action on the cell death induced by reactive oxygen species (ROS) (Non Patent Literature 10). As described above, GST-π among GST is understood to have various features and functions.
It is reported that when siRNA against GST-π is allowed to act on a cancer cell system having a KRAS mutation, Akt activation is suppressed and autophagy increases but the apoptosis induction is about moderate (Non Patent Literature 11). Patent Literature 1 discloses that cancer cell apoptosis can be induced using a drug suppressing GST-π and an autophagy inhibitor such as 3-methyladenine as active components. Patent Literature 2 further discloses that when the expression of GST-π and Akt is simultaneously inhibited, the cell growth is suppressed and the cell death is induced and the autophagy induced by the GST-π expression inhibition is notably suppressed by simultaneously inhibiting the expression of Akt and the like.
However, in the cell having a mutation in the BRAF gene described above, there is no finding in the relationship between the expression of GST-π and the cell growth or the cell death or the role of GST-π in the signal transduction.